High pressure converter for deep well drilling

ABSTRACT

A pressure converter for a drill pipe includes a housing with a header channel therein which is in communication with a drill bit, a drive unit which is driven by a driving drilling fluid flow of the drill pipe, a valve which is operatively connected to and moved by the drive unit, a piston which moves in a reciprocating manner thereby creating a pressure stroke and a return stroke, and a check valve through which a portion of the drilling fluid flow is discharged to the drill bit via the header channel. The reciprocating movement of the piston is controlled by the valve and the piston includes a first piston area which is subjected to the driving drilling fluid flow during the pressure stroke and which is in communication with a returning drilling fluid flow running outside the drill pipe, a second piston area which is opposite the first piston area and which is in communication, during the pressure stroke and the return stroke, with the returning drilling fluid flow, and a third piston area which is opposite to and smaller than the first piston area, and which 1) during the pressure stroke, generates an increased pressure in a portion of the driving drilling fluid flow, and 2) is in communication with the driving drilling fluid flow during the return stroke. The increased pressure portion of the driving drilling fluid flow is discharged via the first check valve and the header channel to the drill bit.

FIELD OF THE INVENTION

This invention relates to a pressure converter for mounting above thedrill bit at the lower end of a drill pipe for deep drilling, inparticular for oil and gas, for the purpose of generating an increasedfluid pressure by utilizing energy in a drill fluid flow downwardsthrough the drill pipe.

DESCRIPTION OF THE RELATED ART

Various proposals are previously known for such utilization of the drillfluid flow, in particular in order to obtain an enhanced or moreefficient drill operation. An example of such known techniques is to befound in the international patent application, PCT/EP82/00147. Thisexample relates to the employment of an impact effect brought about withthe drill fluid flow as a source of energy, so as to enhance thedrilling action.

SUMMARY OF THE INVENTION

Of particular interest to the present invention is the employment of oneor more high pressure jets adapted to make the drilling more effectiveby providing a cutting action in a surrounding rock formation, which ispreviously known per se. The invention, however, is directed to a noveldesign of a pressure converter for generating the required high fluidpressure.

What is novel and specific to the pressure converter according to theinvention in the first place, consists therein that drive means isadapted to be driven by the drill fluid flow and to move valve meanscontrolling piston means for reciprocating movement with a pressurestroke and a return stroke, said piston means having at one side arelatively large piston area adapted to be subjected to the drill fluidpressure in the drill pipe during the pressure stroke, and having at theother side a first, opposite piston area which both during the pressurestroke and the return stroke is subjected to the return pressure in thedrill fluid flow upwards outside the drill pipe, and a second, oppositeand relatively small piston area which during the pressure stroke isadapted to provide an increased pressure in a smaller proportion of thedrill fluid flow, whereby a check valve provides for discharge of thissmaller proportion of the flow to a header channel leading forwards tothe drill bit, whereas the large piston area during the return stroke isadapted to be subjected to the return pressure outside the drill pipeand the small piston area to the pressure in the drill pipe.

As a typical example it may be mentioned that the pressure in the drillfluid flow which is employed, can be about 200-300 bar, whereas thesmaller flow which is converted can obtain an increased pressure of forexample 1500-2000 bar. (When here and in the following descriptionexamples of figures referred to pressure magnitudes are given, these arein the principle relative magnitudes, i.e. pressure differences, sincethe static pressure determined by the depth concerned has beenneglected.) The resulting high pressure fluid is led to nozzles in thedrill bit, from which it is emitted in the form of powerful jets beingable to cut into the surrounding rock and thereby release stresses inunderlying masses. This facilitates the drilling operation and speeds upthe drilling.

In the new pressure converter described here it may be an advantage toprovide a spring for assisting at least initially during the returnstroke, preferably a compression spring which acts against the first,opposite piston area.

Moreover, the piston member can be freely movable in its axial directionunder the influence of the drill fluid and spring pressures mentioned,and besides the reciprocating movement of the piston preferably takesplace in the longitudinal direction of the drill pipe.

In most applications it is preferred, according to the invention, thatthe header channel for the high pressure flow is arranged to bethrough-going from one to the opposite end of the pressure converter, inorder to make possible a coupling to similar pressure converter units atboth ends, so that there is formed a common header channel for severalpressure converter units constituting a group, for example consisting of15 to 20 units. This will increase the total capacity in providing thedesired high pressure fluid flow. Moreover, there is obtained asubstantial advantage by phase-shift of the pressure strokes in theindividual units in such a group, in order thereby to obtain a total,smooth high pressure flow. Finally, it is an advantage with such a grouparrangement that in the case of failure in one or a small number ofpressure converter units, the remaining units in the group will be ableto supply a sufficient amount of high pressure fluid for the applicationconcerned. In other words, the pressure converter units in the group arestanding in a parallel relationship to each other with respect to thedrill fluid flow.

The pressure converter according to the invention will be able tooperate exclusively under the direct influence or control through thenormal fluid flow from pumps at the top of the drill string, so that itis not necessary to provide specific control systems or connections inorder to regulate the generation of the desired high pressure flow ofdrill fluid. By increasing the pressure, the velocity and/or the amountof drill fluid being supplied by the pumps, the pressure converter unitswill give a high pressure flow having a larger or smaller magnitude, anda higher or lower pressure respectively. Commonly employed means forcontrolling the drill fluid flow downwards from the top of the drillstring, will be useful in this connection. The drill fluid from thepumps which typically provide a pressure of 200 to 340 bar, thus, flowsdownwards within the drill string or the drill pipe, whereby a mainportion is led directly to the drill bit, whereas a smaller proportionof the drill fluid flow passes through the pressure converter units forconversion to the desired higher pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall be explained more closely in the followingdescription, with reference to the drawings, in which:

FIG. 1 is a highly schematic flow diagram showing, among other things,typical pressure relationships in connection with a drill stringprovided with pressure converters according to the invention,

FIG. 2 shows in partial cross-section a practical embodiment of apressure converter according to the invention,

FIG. 3 shows the pressure converter of FIG. 1 with internal parts,including movable parts removed,

FIG. 4 in partial cross-section shows a cover being provided at the topof the converter unit in FIG. 2,

FIG. 5 shows in plane view a plate shaped valve member incorporated intothe pressure converter unit in FIG. 2,

FIG. 6 shows a cross-section according to the line A--A in FIG. 2,

FIG. 7 shows an assembly of four pressure converter units according toFIG. 2, in a group provided with a top piece and a bottom piece,

FIGS. 8A and 8B more in detail show the top and the bottom of the groupin FIG. 7 when mounted in a drill pipe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Main features of what takes place in a drill string and accompanyingtypical examples of pressure relationships when using pressureconverters according to the invention for conversion from fluid having arelatively low pressure of about 200 to 340 bar to a smaller amount offluid having a high pressure of about 1500 to 2000 bar (relativemagnitudes), are shown in FIG. 1.

A fluid flow A comes from a pump system resulting in a pressure of about200 bar and a maximum of 340 bar, and an amount of about 2000 to 4000liters per minute, depending upon the length of the drill string and thecapacity of the system. The drilling fluid enters a pressure convertergroup having four units, where it passes by a turbine B for valveoperation. There is an estimated to be a pressure drop of about 50 barwhen passing through the drill string and the turbine.

The drilling fluid is subdivided into two flows. One of about 400 to 600liters per minute goes through the pressure converters, whereas theremaining part goes through the system to the drill bit where, becauseof jet nozzles, there is a pressure drop of about 180 to 270 bar. Afterpassage by the drill bit there is a return flow H having a pressure dropof about 20 bar before the drilling fluid returns to the drilling moduleat the top of the drill string, where the flow in the usual manner isled into an open tank (1 bar). In each pressure converter the fluid flowC will perform its work by increasing the pressure in a smallerproportion of the drilling fluid, and thereby the pressure in this flowdrops from about 200 to 290 bar to about 20 bar. Then the flow passesthrough a tube D and out into the return flow H, which runs at theoutside of the drill string or pipe inside the usual casing and atpressure of about 20 to 40 bar.

The smaller portion of the fluid flow to which energy has been added,has been subjected to a pressure increase from about 200 to 290 bar toabout 1500 to 2000 bar. This fluid flow is now led through a channelsystem E down to the drill bit. In parts of the drill bit there aremounted specific high pressure nozzles which make it possible to "cut"into the formation. The counter-pressure is the same as for the drillingfluid, about 20 bar, and there is a pressure drop across these nozzlesof about 1500 to 2000 bar minus 20 bar, which gives about 1480 to 1980bar. The flows F and G combine and convey crushed and loose particles tothe surface, i.e. flows F and G are incorporated into the total returnflow H.

The embodiment shown in FIG. 2 in the first place comprises a generallycylindrical housing 10 adapted to receive a piston 6 which has threeoperative piston areas, namely an upper, relatively large piston area11, a first, opposite piston area 13 and a second opposite andrelatively small piston area 12 at the lower end of piston means 6. Thepiston 6 is adapted to be freely movable axially under the influence ofvarying drilling fluid pressures on the respective piston areas, as wellas under the influence of a compression spring 14 engaging the pistonarea 13.

As will appear from the following description, the space or volume 31above piston area 11 can be denoted a low pressure space, whereas volume32 below piston area 12 correspondingly can be denoted high pressurespace. Through a check valve 15 this latter space is connected to aheader channel 16 for the resulting drilling fluid flow at an increasedpressure. The channel 16 runs through the housing 10 in the wholelongitudinal direction thereof for the purpose of interconnectingseveral such pressure converter units to a group. Such a grouparrangement shall be discussed more closely below with reference toFIGS. 7 and 8.

Diametrically opposite in relation to the header channel 16 there isalso through the whole length of housing 10 provided a widened wall parthaving a bore for a through-going drive axle 21 which at its ends hasmeans intended for coupling to corresponding pressure converters at bothends. The drive axle has a small gear 25 which via a second (not shown)small gear on an axle 24, serves to rotate a valve member in the form ofa round plate 27 having teeth around its circumference as shown moreclearly in FIG. 5. During operation of the pressure converter, the valveplate 27 is adapted to rotate continuously about the longitudinal axisof the pressure converter unit, which axis normally will coincide withthe axis of the drill pipe in which the pressure converter is mounted.

The valve plate 27 mentioned above constitutes an essential component ofvalve means which serve to direct a portion of the drilling fluid flowinto and out of the space 31 above the piston area 11. This valve,moreover, at the top of housing 10 comprises a cover 22 which has twochannels positioned substantially opposite to each other, i.e., an inletchannel 34 and and outlet channel 35, both of which continue through thepiston housing wall, as seen at 34 in FIG. 2. The cover 22 is also shownmore in detail in FIG. 4. See also FIG. 3 as far as the extension ofchannels 34 and 35 through the piston housing wall is concerned. Furtherradially out from channel 35 the outlet continues through a short tube(not shown) to the annulus for the return flow between the drill stringor tube and the casing.

The inlet channel 34 in cover 22 leads inwards to an arcuate slit 22A,whereas the outlet channel 35 in a corresponding manner communicateswith an arcuate slit 22B. Both slits are open downwards in order tocooperate with a through-opening 27B in valve plate 27 during rotationthereof.

It may be an advantage to provide a bearing plate 26 having similarslits as in the cover, between valve plate 27 and cover 22. A similarbearing plate or sealing plate 28 is mounted underneath valve plate 27and has corresponding arcuate slits as in plate 26 and cover 22. Thecomplete valve means with cover 22 on top and sealing plate 28 at thebottom, is maintained in place first by an upper locking ring 23 andsecond by a lower locking or sealing ring 29. Besides, there is shown acentral bolt at 30, which among other things constitutes the axle forthe rotation of valve plate 27, whereas the other plates in the valvemeans are stationary. The various plates incorporated in the valvedesign can be made of different materials, but in order to withstand thetough environment which is represented by the circulating drillingfluid, it may be an advantage to employ high quality materials, possiblyin the form of surface coatings, for example ceramic materials, which inparticular can be of interest for the two bearing plates 26 and 28.

In FIGS. 2 and 3 there are further shown (three of a total of four)short tubes or connections 37A, 37B and 37C for putting the space abovethe first, opposite piston area 13 in fluid communication with thereturn passage-way for the upwardly running drilling fluid in theannulus between the drill tube or string and the well casing. Thus, thespace in front of piston area 13 will all the time be subjected to arelatively low drilling fluid pressure.

The cross-section in FIG. 6 shows in more detail the high pressure space32 which in addition to an outlet through the check valve 15 to headerchannel 16 for high pressure fluid, has two inlets with respectiveassociated check valves 39A and 39B which makes possible inflow ofdrilling fluid from the main flow thereof inside the drill pipe.

The operation of the pressure converter as described is as follows:

Starting from an upper dead-point of the piston 6 a pressure stroke inthe downward direction is performed when the through-opening in valveplate 27 moves underneath inlet slit 22A in valve plate 22, wherebydrilling fluid at a pressure of about 200 to 300 bar enters throughinlet channel 34 and exerts a downwardly directed drive force on pistonarea 11. The opposite piston area 13 is subjected to a much lowerpressure, typically about 20 to 40 bar, whereas spring 14 can have apushing force of for example 2 to 400 kg. The driving force downwards atthe upper side of the piston 6, however, will override the counterforceat the underside and will bring about the desired pressure stroke.During this downward movement drilling fluid in front of the oppositepiston area 13 will be pressed out through tube connections 37A, 37B and37C at the same time as spring 14 is compressed and partly received inthe annular recess in which the spring is held. An abutment at the topof the recess (see FIG. 3) can serve to limit the maximum downwardmovement in the pressure stroke.

The intended buildup of a high pressure takes place in space 32 in frontof the small piston area 12 at the bottom of the converter unit, anddrilling fluid under high pressure is pressed out through check valve 15to the header channel 16.

The angular extension and the separation of the two separate slits 22Aand 22B in cover 22, as well as the associated slits positionedessentially quite correspondingly in the bearing plate 26 and 28,together with the design of through-opening 27B in valve plate 27,determine the development of the pressure stroke described above and thedevelopment of a return stroke which brings the piston means from thebottom position or the lower deadpoint in an upward direction towardsthe top position which is the starting point of the pressure stroke.

The return stroke is initiated when the opening 27B in the valve plate27 through the outlet channel 35 puts the space 31 in communication withthe annulus between the drill tube and the casing, i.e., with thementioned much lower pressure in the return flow of drilling fluid.Then, in the first place the pressure on piston areas 11 and 13 will beequal, and the compression spring 14 provides for initiating the upwardmovement of the piston means. At this phase there will still exist arelatively high pressure in space 32 in front of the small piston area12, typically a pressure somewhat below 1500 bar, which also contributesto the upward piston movement. Valve 15 will close for the establishedhigh drilling fluid pressure in header channel 16. As the piston movesupwards space 32 will expand and inlet valves 39A and 39B (FIG. 6) willopen for the drilling fluid pressure in the drill pipe, typically about200 to 300 bar. This will also contribute to the total upwardly directedpushing force. During this return stroke there will be an inwarddrilling fluid flow through tube connections 37A, 37B and 37C into thespace in front of piston area 13.

In connection with the operation as described here, it will be realizedthat the spacing between the ends of through-slits 22A and 22B and thecorresponding slits in plates 26 and 28, must be sufficiently large inrelation to the size of opening 27B in valve plate 27, in order toprevent any direct through-flow or "short circuit" from the highdrilling fluid pressure to the return flow pressure.

Above there has been described a single pressure converter unit and theoperation thereof. With reference to FIGS. 7 and 8 it shall now beexplained how such converter units can be assembled into a group, interalia for obtaining a total higher yield or capacity.

FIG. 7 shows four pressure converter units 41, 42, 43 and 44 beingcoupled together end to end in the longitudinal direction, with a toppiece 3 mounted on unit 41, whereas a bottom piece 5 is mounted on unit44. At converter unit 41 there are indicated short tubes 37A and 37B asin FIGS. 2 and 3, as well as the drive axle 21 which is rotationallycoupled to the drive axle of the remaining units, i.e., axles 21A, 21Band 21C respectively.

The top piece 3 carries drive means in the form of a turbine 20 adaptedto be driven by the drilling fluid flow, whereby a gear transmissionconveys the power from the turbine axle to the assembled drive axles forrotating these in common and thereby provide for the intended control ofthe valve means in the converter units. It is an advantage to have thesephase shifted, i.e., with mutual angular displacement, so that thepressure strokes and thereby the high pressure output from each of theunits to the common header channel are smoothed to a more constant highpressure flow than will result from each individual pressure converter.At 46 the header channel is extended into the bottom piece 5 which has acentral outlet for further fluid flow to the region at the drill bit(not shown).

The assembled group of pressure converters is mounted free-standing inthe drill pipe supported by the bottom plate. FIG. 8 shows some detailsin this connection, at the top and the bottom of the group respectively.Converter units 41 and 44 are shown completely, whereas units 42 and 43are shown only in part. The surrounding drill pipe 1 forms an annularfluid passageway 40 outside and surrounding the pressure converter unitsin the group, so as to make possible a normal movement of the mainportion of the drilling fluid flow down to the drill bit. The totaldrilling fluid flow from above is indicated with arrow 19 in FIG. 8A.Through a narrowed inlet part at the inside of drill pipe 1, thedrilling fluid flow is led against an impeller 20 located upstream inrelation to the converter group. The previously recited short tubes ortube connections out to the annulus outside the drill pipe 1, of which atube 37 is indicated in FIG. 8A, as the case may be can contribute tothe anchoring and aligning of the whole converter group within drillpipe 1. This annulus for the return flow of drilling fluid is indicatedby reference numeral 50.

Even though each individual pressure converter alone can have a toosmall capacity with respect to its discharge of high pressure fluid, inrelation to the actual requirement, the assembly into groups asdiscussed above will make it possible to obtain a sufficiently largecombined yield. Each individual pressure converter unit will have acapacity (liters per minute) which also depends upon the stroke rate ofthe piston means. A factor in this connection, and of significance forthe operation as a whole, is that the turbine 2 with its impeller 20 isnot required to have any particularly high power output, since thepurpose thereof only is to move the valve means which controls thedrilling fluid flows into and out of the piston means, which is the partof the structure which must have comparatively high power capacity.

An assembled group of for example 15 to 20 converter units in practicecan have a total length of about 6 meters and can be mountedfree-standing on a bottom piece within a section of drill pipe or drillstring having a corresponding length, possibly with strut elementsbetween the inside of the drill pipe or string and the pressureconverter units. For additional increase of capacity, several suchsections or lengths of about 6 meters can be interconnected.

Since there is no need for any direct connection from the pressureconverters to the surface, for example a drill rig, apart from thedrilling fluid flow which is supplied by the common drilling fluidpumps, the control and regulation of the pressure converting operationmust be built up with due consideration thereof. A relatively importantfactor in this connection is the pressure drop across the drill bitduring operation. Prior to a drilling operation with accompanyinggeneration of high pressure drilling fluid as described above, it willbe near at hand and normal to carry out the following:

Adjustment of permanently mounted nozzles in the drill bit fordetermining the pressure drop depending on the drilling fluid flow topass by.

Adjustment or setting of pressure drop in the drilling fluid supply tothe pressure converters as well as the pressure drop in the return flowof drilling fluid.

Pressure drop across the turbine which provides for valve movement.

Variable parameters which have influence on the pressure conversionprocess are the flow velocity and volume as well as the pressure. Thereturn pressure may also be a parameter which it is desirable to vary inorder to control the process in the converter units.

Theoretically one should be able to determine the pressure increase andthe volume in the fluid converter by proceding as follows:

By increased velocity of the fluid the turbine for valve operation willhave an increased rate of rotation, and the same applies to the rate ofalternations in the valve system. This will increase until reaching amaximum for input or output respectively of fluid in the individualunits and piston movement.

By increasing or reducing the pressure from the pumps the pressure dropacross the drill bit will increase or decrease respectively, and therebythe resulting pressure in the high pressure fluid supplied, willincrease or decrease respectively.

Even though the pressure converter described is primarily intended forsupplying high pressure fluid to jet nozzles for cutting in rock, thereare also possibilities of different applications of such drilling fluidunder an increased pressure, for example for driving particular drillingdevices.

Among possible modifications within the framework of the invention, itis mentioned that the cooperating openings and slits in the valvemember, bearing plates and cover can be arranged "inversely" in relationto the example shown, i.e. with a small angular extension of the slitsin the cover and the bearing plates, whereas the opening in the valvemember can have a more extended slit shape with a larger angularextension about the central axis.

We claim:
 1. A pressure converter for a drill pipe having a drill bit,the pressure converter comprising:a housing including a header channelin communication with the drill bit; a drive unit which is driven by adriving drilling fluid flow of the drill pipe; a piston which moves in areciprocating manner thereby creating a pressure stroke and a returnstroke, said piston includinga first piston area which is subjected tosaid driving drilling fluid flow during said pressure stroke and whichis in communication, during said return stroke, with a returningdrilling fluid flow running outside the drill pipe, a second piston areawhich is opposite to said first piston area and which is incommunication with, during said pressure stroke and said return stroke,the returning drilling fluid flow, a third piston area which is oppositeto and smaller than said first piston area and which 1) during thepressure stroke, generates an increased pressure in a portion of thedriving drilling fluid flow, and 2) is in communication with the drivingdrilling fluid flow during the return stroke, a valve which isoperatively connected to and moved by said drive unit, said valvecontrolling the reciprocating movement of said piston, and a first checkvalve through which said portion of the drilling fluid flow isdischarged to the drill bit via said header channel.
 2. A pressureconverter according to claim 1, further comprising a spring which exertsa force against said piston to assist movement of said piston during atleast a portion of said return stroke.
 3. A pressure converter accordingto claim 2, wherein said spring is a compression spring.
 4. A pressureconverter according to claim 1, further comprising at least a secondcheck valve, and wherein said housing has a first space therein which islocated below said third piston area and which is connected to a fluidpassageway of the drill pipe via said second check valve, and saiddriving drilling fluid flows through said fluid passageway.
 5. Apressure converter according to claim 1, wherein said piston is freelymoveable along its longitudinal axial direction due to a pressure of thedriving drilling fluid flow.
 6. A pressure converter according to claim1, wherein said reciprocating movement of said piston is along thelongitudinal direction of the drill pipe.
 7. A pressure converteraccording to claim 1, wherein said header channel extends from a firstend of said housing to a second end of said housing such that saidhousing can be coupled at said first and second ends to additionalpressure converters, whereby said header channel and correspondingheader channels of said additional pressures converters form a commonheader channel.
 8. A pressure converter according to claim 1, furthercomprising a plurality of tube connections, and wherein said housingincludes a second space below said second piston area, and saidplurality of tube connections each extend from said second space to anannulus outside of the drill pipe thereby at least partially anchoringthe pressure converter within the drill pipe.
 9. A pressure converteraccording to claim 1, wherein said drive unit includes a turbine with animpeller, and at times when said impeller is subjected to the drivingdrilling fluid flow, the impeller rotates.
 10. A pressure converteraccording to claim 1, further comprising a drive axle which extendsthrough a wall of said housing and which is capable of being coupled toa corresponding drive axle of another pressure converter, and whereinsaid drive axle moves said valve.
 11. A pressure converter according toclaim 1, wherein said valve includes a plate shaped member having acentral axis about which it rotates, said central axis coinciding with alongitudinal axis of the drill pipe.
 12. A pressure converter accordingto claim 11, wherein said valve member includes a plurality of teethalong its circumference which are rotatably connected to said driveaxle.
 13. A pressure converter according to claim 12, further comprisinga first gear connected to said drive axle and a second gear disposedbetween and operationally connected to said plurality of teeth and saidfirst gear.
 14. A pressure converter according to claim 11, wherein saidvalve member has a through-opening therein which directs the drivingdrilling fluid flow from the drill pipe to said piston and returns thedriving drilling fluid flow to the outside of the drill pipe.
 15. Apressure converter according to claim 14, further comprising a generallyplate shaped cover having a first radial channel forming an inlet, asecond radial channel forming an outlet, an inlet slit in communicationwith said inlet, and an outlet slit in communication with said outlet,and wherein said driving fluid flow enters from the drill pipe throughsaid inlet and said inlet slit and returns to an annulus outside of thedrill pipe via the outlet slit and the outlet, and said inlet and outletslits have an angular extension relative to the central axis which islarger than an angular extension of the through-opening of the valvemember relative to the central axis.
 16. A pressure converter accordingto claim 15, further comprising first and second wear resistant bearingplates respectively disposed on opposite sides of said valve member,said first and second wear resistent bearing plates each havingthrough-going slits therein which respectively generally correspond tosaid inlet and outlet slits.
 17. A pressure converter according to claim14, further comprising a generally plate shaped cover having a firstradial channel forming an inlet, a second radial channel forming anoutlet, an inlet slit in communication with said inlet, an outlet slitin communication with said outlet, and first and second wear resistantbearing plates that are disposed respectively on opposite sides of saidvalve member, said wear resistant bearing plates each havingthrough-going slits which respectively correspond to the inlet andoutlet slits, and wherein said driving fluid flow passes through saidinlet and said inlet slit and returns to an annulus outside the drillpipe via the outlet slit and the outlet, and said inlet and outlet slitsand said through-going slits have an angular extension relative to thecentral axis which is smaller than an angular extension of thethrough-opening of said valve member.
 18. A pressure converter accordingto claim 11, wherein said piston reciprocatingly moves due to varyingdriving drilling fluid flow pressures which ar respectively exerted onsaid first, second and third piston areas.
 19. A pressure converteraccording to claim 18, further comprising a spring which assists in thereciprocating movement of said piston.
 20. A pressure converteraccording to claim 7, wherein each of said addition pressure convertersare substantially the same as said pressure converter and thereciprocating movement of the respective valves of the pressureconverter and said additional pressure converters are phase shifted tosmooth out a total driving drilling fluid flow through said commonheader channel.